1. Field of the Invention
The present invention relates to a device for cross-level digital signal transmission. In particular, the present invention relates to a device capable for transforming voltage levels of digital signals.
2. Description of the Prior Art
A battery state monitoring circuit of a battery module management system comprises a battery monitoring chip capable of outputting digital data. Because additional series of battery units are used in applications requiring more power, a single battery monitoring chip cannot monitor the voltage or temperature of each battery. More battery monitoring chips are required to correspondingly monitor the state of related batteries. In addition, it is necessary to use a central control microprocessor to acquire status of all the batteries in some complicated battery management functions. Because of the different electrical ground levels in each battery monitoring chip, these chips have problems transmitting battery status data to the central control microprocessor.
In general, a serial data transmission interface standard of a bi-directional signal line is designed according to requirements of communication between a first digital system 1 and a second digital system 2, both connected to the same ground. The connection between the first digital system 1 and the second digital system 2 is shown in FIG. 1, wherein the output stages of both systems are open-collector NPN transistor (101 and 102). The first digital system 1 includes an input unit 501 and output unit 502, and the second digital system 2 includes an input unit 503 and output unit 504. When any one of systems does not transmit data, transistors 101 and 102 are turned off, and a voltage level of a transmission line is H through a voltage source Vcc and a pull-up resistor Rpull-up. When one of the two systems transmits data, the transistor of this system is open, and the voltage level of the transmission line is L. However, if there is a voltage difference Vdiff in ground level between two systems, the digital signal transmitted between the two systems is in error. As shown in FIG. 2, voltage Vg1 is the ground level in the first digital system 1, and Vg2 is the ground level in the second digital system 2. Here, Vg2=Vg1+Vdiff, and Vdiff>Vcc. When the voltage difference between two systems is greater than the pull-up voltage of the pull-up resistor Rpull-up, the logic level of the signal transmitted to the second digital system 2 from the first digital system 1 is regarded as L, and the logic level of signal transmitted to the first digital system 1 from the second digital system 2 is regarded as H. It is thus unable to transmit correct signals. Here, the labels L and H represent low voltage and high voltage levels, respectively.
As a result, a general solution is to add a control line to either the first digital system 1 or the second digital system 2, which transmits data, to control the direction of data transmission. As shown in FIG. 3, a first logic level converter 3 and a second logic level converter 4 comprise signal control terminals 103 and 104, respectively. The first logic converter 3 is a cross-level buffer to shift digital signal from a low voltage system to a high voltage system, and the logic level converter 4 is a cross-level buffer to shift digital signal from a high voltage system to a low voltage system. Thus, connection of an input and output between the first logic level converter 3 and second logic level converter 4 forms a positive feedback. If signal control terminals 103 and 104 do not turn off one of the two systems, the locking data property will hinder transmission. Thus, another control signal is required to determine allowed data direction at any time, as the T/R signal in FIG. 3, and then data of two systems can transmit effectively. The above-mentioned solution does not, however, solve the problems described in FIG. 2 completely. For example, first, it is necessary to generate the added control signal by a certain circuit function block or programming control circuit, such that the circuit becomes more complex; second, the symmetry of the two systems signal is affected. In FIG. 3, the first digital system 1 must set the data direction as a correct direction when the second digital system 2 starts to transmit data, and the data can reach the first digital system.